The suspect, Prajak Masawaeng, has an appalling criminal record of theft and attempted murder, police said.
"He is not insane, but he has a criminal instinct," said Pol Maj Gen Surapol Wiratkosin, deputy commissioner of the Provincial Police Region Two, who is in charge of the case.
"He poses a huge danger to society. When he drinks, it aggravates his anger. When he is drunk, he cannot control his temper and is ready to commit a crime."
Mr Prajak, 19, allegedly killed an 18-year-old woman and two boys, aged seven and two, at a house in the Paradise Hill 2 housing estate on Bunsamphan Road in Bang Lamung district of Chon Buri on Jan 10.
The young woman was the house owner's niece and the boys were the owner's children.
Mr Prajak went to the house to wait for his girlfriend, who had gone outside with the house owner, Wilaiwan Liberman, 32, the mother of the two boys.
Ms Wilaiwan was talking to Mr Prajak's girlfriend about getting her a job as a dancer in Pattaya.
Mr Prajak told police he approached Ms Wilaiwan's niece, who was minding the children, and asked her if he could wait inside the house.
The niece, he said, was impolite to him. In a fit of rage, he knocked her unconscious and stabbed her and one of the boys to death.
The other boy woke up, so he stabbed and killed him too. He then fled the house.
Police sought a warrant for Mr Prajak's arrest and launched a manhunt, which pressured the suspect to turn himself in. He had fled to his home in Kanchanaburi's Tha Muang district, where he surrendered to police.
Pol Maj Gen Surapol also learned Mr Prajak had faced arrest warrants for attempted murder, theft and illegal detention in Kanchanaburi.
On Jan 1, he and two others allegedly attacked Siam Chumpengpan, 18, in Tha Muang district and escaped with his girlfriend.
The three slashed Mr Siam with a long knife as he was entering his rented room with a young woman.
Mr Siam, who is still in hospital for treatment of multiple slash wounds, said the attack was unprovoked and that he never had any conflict with Mr Prajak before.
After the assault, Mr Prajak fled with his girlfriend to Pattaya, where they stayed with his relative.
Later, they were introduced to Ms Wilaiwan, who said she could help his girlfriend find a dancing job.
"The case is very disturbing. The suspect has a long record of crimes and is dangerous," Pol Maj Gen Surapol said.
Before Mr Prajak's arrest, Pol Maj Gen Surapol interviewed Ms Wilaiwan and her partner, Jason Liberman. Mr Liberman asked police to arrest the killer quickly, Pol Maj Gen Surapol said.
He noted Mr Prajak, who has been charged with murder, may have tried to rape Ms Wilaiwan's niece. He said that may have been another reason he was driven to kill, although his statements suggested he attacked her in anger.